Add JSON read/write support

[AtharvaKhare]

​DataFrameJsonReader and ​DataFrameJsonWriter need to be implemented, along with its tests, and examples. We can use NeoJSON library for this purpose. It will make using internet APIs as data-source easier.

Related issue: SquareBracketAssociates/Booklet-DataFrame#1

[khinsen]

That's a very good idea. The JSON format should be compatible with the one used by R as much as possible. At least for numerical and string data, but ideally also for dates and some other common data types.